The field of the invention is in an improved process for producing aluminum nitride needles and particularly a process using moderate temperatures (950.degree.-1000.degree. C.) and inexpensive, low toxicity reactants (i.e. NH.sub.3 & Al.degree.) for the production of the AlN needles.
In the prior art several methods have been used for the production of AlN needles. In one of these processes such as is shown in U.S. Pat. No. 3,653,830 there is taught the heating of a mixture of aluminum nitride powders and aluminum powders to 580.degree.-1000.degree. C. in an atmosphere consisting of nitrogen or ammonia gas with additions of halogens which create an exothermic nitrification of aluminum heating the charge to above 2000.degree. C. and forming aluminum nitride fibers. While this may appear attractive from an economic point of view, the very high toxicity of the halogens needed in the reaction preclude their use because of the need for special handling and equipment.
Another prior art method of producing AlN needles involves the reduction of Al.sub.2 O.sub.3 with carbon in an atmosphere of nitrogen at 1600.degree.-1700.degree. C. While there are attractive aspects to this process Al.sub.2 O.sub.3 (s)+3C.degree.(s).fwdarw.2AlN.sub.(s) +3CO(g) because all the reaction products are gases except for the AlN, the temperatures required are sufficiently high to make the process industrially unfeasible.
In the present invention the process uses a common low cost tube furnace with an inexpensive ceramic boat as the reaction container. The ceramic boat is charged with aluminum powder and placed within the tube furnace so that one end of the boat is at the leading upstream end of the hot zone of the furnace. Ammonia and argon are mixed and introduced into the tube and at operating temperature of 950.degree.-1000.degree. C. the reaction proceeds smoothly, the argon being a carrier for the ammonia to retard the reaction and allow the needles the necessary time to grow.